The present invention relates to heat transfer systems, and more particularly to a system for heat transfer employing an electrokinetic pump.
Heat reduction systems typically remove heat from a source at an elevated temperature such as, for example, a computer processor, and exhaust the heat from the source to a heat sink having a lower temperature, such as ambient air temperature. By this heat transfer process, the source of heat is maintained at a finite temperature above the sink temperature. Modern electronic systems typically contact a finned heat sink to the source of heat and flow air across the heat sink to remove excess heat. Faster processors and more high power electronics in today's computer cases generate more heat and raise the air temperature within the computer case substantially higher than ambient air temperature. This makes the sink temperature (i.e., the air within the case) higher and thus raises the operating temperature of the components within the case.
The quest for more compact electronics such as very slim laptop computers, leaves little room within the case through which air can flow, thereby making it difficult to achieve the air flow rates needed to adequately cool the components. There is therefore a need for improved methods of cooling electronic systems and other components. Preferably, improvements will not rely on air moving through the electronics enclosure, but rather will remove heat from a source and transfer it to a case-mounted external exchanger, such as a fin, where the heat can be exchanged with lower temperature ambient air.
It is known to use heat pipes to assist in heat exchange. Typically a heat pipe is a sealed conduit partially filled with liquid that has a wicking structure along the inside walls. One end of the heat pipe is heated and the other end cooled. Liquid evaporates from the heated end, and the resulting vapor flows down the core of the heat pipe to be condensed at the cooled end. Liquid is resupplied to the heated end by wicking along the conduit walls. While heat pipes are attractive, because they are fully sealed and have no mechanical moving parts, they have limitations such as limited heat flux capacity, sensitivity to orientation and the need for a rigid conduit. Additionally, the wick structure can be expensive and hard to manufacture reliably.
A known alternative to the heat pipe is a capillary-pumped-loop, also called the pulsating-heat-pipe or “PHP.” This alternative is described in U.S. Pat. Nos. 4,921,041 and 5,219,020. The pulsating heat pipes are used in a fashion similar to a conventional heat pipe. The pulsating heat pipe is a conduit that is sealed and is partially filled with liquid. The pulsating heat pipe is different than a conventional heat pipe in that the pulsating heat pipe does not employ an internal wicking structure. The liquid within the conduit is naturally distributed as liquid-vapor slugs. In application, a portion, or portions, of the pulsating heat pipe is heated and another portion, or portions, of the pulsating heat pipe are cooled. The production of vapor at the heated portions raises the pressure locally and this causes motion of the liquid slugs along the conduit. This motion serves to move the liquid from heated to cooled terminals of the device. However, as with heat pipes, there are limitations with maximum heat flux, a strong sensitivity to orientation, and a strong sensitivity to the amount of liquid inside of the conduit.
Additionally, it is known to use flowing liquid cooling loops. However, a flowing liquid system requires a mechanical pump that adds size, weight, annoying noise and vibration and that can be prone to premature failure and/or leakage. Thus, there remains a need for an improved cooling system that can transfer heat from a heat source to an external heat exchanger.